In Japan, the first cause of death is cancer, and cancer is steadily increasing. In recent Japan in which improvement in quality of life (QOL) is needed, therapy using radiation attracts attention as a cancer therapy method. In order to improve the QOL as the need, a radiation cancer therapy technique which is a seed becomes highly accurate, and radiation cancer therapy also starts to be widespread in Japan.
Radiation used for therapy includes an X-ray, a particle beam (a proton beam or a heavy particle beam), an electron beam, and a neutron beam. Particularly, in recent years, a particle beam therapy apparatus using a proton beam and a heavy particle beam therapy apparatus using a heavy particle beam (for example, a carbon beam) have been remarkably developed. A patient is irradiated with a particle beam by using the property that the proton beam and the heavy particle beam generate a dose peak (black peak) by being intensively applied with energy immediately after being stopped, and thus a dose can be applied to an affected part of cancer in a concentration manner, so that low invasive and highly accurate cancer therapy can be expected.
Also in cancer therapy using an X-ray, intensity-modulated radiotherapy (IMRT) and image-guided radiotherapy (IGRT) have been developed, and an effort to cause a dose in X-ray irradiation to concentrate on an affected part of cancer has been made. In accordance with sophistication of a radiation therapy apparatus, there is the need for improvement of the whole accuracy related to radiation therapy, such as the accuracy of a therapy plan and the accuracy of patient positioning, dose rate measurement for quality assurance (QA) of a therapy plan and a therapy apparatus.
In radiation therapy, an ionization chamber of which stability and reproducibility are favorable are widely used to measure a dose rate of radiation applied to a patient. However, the ionization chamber has a limit in miniaturization due to a detection principle thereof, and, instead thereof, a dose distribution measurement using a semiconductor detector which is relatively easily miniaturized is performed. In a case where even a signal processing system is included, the semiconductor detector also has a limit in miniaturization. Since a high voltage is required to be applied in such a radiation detector, it is difficult to insert the radiation detector into a patient's body, and to measure a dose rate. Such a detector generally has high density, has a greater interaction with radiation than a substance in the body and water, and thus the influence of the radiation detector cannot be disregarded.
As described above, in a situation in which an actual internal absorbed dose cannot be understood, a dose distribution of an affected part obtained through therapy planning has a margin by taking into consideration body motion of the patient due to respiration or the like. This is a cause of reducing the irradiation accuracy of radiation to an affected part. In the body of a patient, in a case where a normal part sensitive to radiation is present near an affected part which is a therapy target part, radiation therapy of the affected part is difficult.
In a radiation therapy apparatus disclosed in JP-A-2003-210596, radiation transmitted through a patient is detected by a radiation detector disposed outside the body of the patient irradiated with the radiation, and, in a case where there is body motion of the patient due to respiration or the like, there is a possibility that an accurate internal absorbed dose cannot be measured. Temporal changes of a position of an organ (affected part) in the body and a size of the organ in a radiation irradiation direction between the time of therapy planning and the time of therapy execution on the affected part using irradiation with radiation, and patient positioning during therapy also cause errors. An internal dose distribution of the patient is estimated through calculation using a dose which is obtained on the basis of a radiation detection signal output from the radiation detector outside the body. A calculation error in this estimation cannot be disregarded.
In order to reduce such errors, a radiation detector is preferably inserted into the body. A radiation detector inserted into the body is disclosed in JP-A-2001-56381. JP-A-2001-56381 discloses a technique in which a scintillation fiber and an optical transmission fiber are inserted into the body, and thus contribution of Cherenkov light which is noise can be removed such that a true radiation dose can be measured.
“Bragg Curve Measurement in Near-Infrared Single Photon Counting Mode”, Katsunori UENO and others, the 110th Japanese Society of Health and Medical Sociology, Vol. 35, Supplement No. 3 (September, 2015), page 77 discloses an optical fiber type online dosimeter (internal dosimeter) which can measure an irradiation dose applied to a patient during proton therapy. The optical fiber type online dosimeter uses Nd:YAG for a detection unit, and performs single-photon counting on near-infrared light generated by Nd:YAG.
“Current status and vision of study for severe accident instrumentation system, 1. Optical fiber-type radiation monitor system”, Takahiro TADOKORO and others, 2015 Annual Meeting of the Atomic Energy Society of Japan Proceedings, Lecture No. 117, issued on Mar. 5, 2015, discloses an optical fiber type radiation monitor, applied to a nuclear power plant, is configured with a detection unit, an optical fiber unit, and an optical measurement unit using Nd:YAG. The optical fiber type radiation monitor can measure a dose rate with the accuracy equal to or lower than ±4% FS in a range of a dose rate of 1.0×10−2 to 9.54×104 Gy/h.
In radiation therapy using a radiation therapy apparatus, when radiation is applied, it is necessary that a dose in a normal tissue near an affected part which is an irradiation target is reduced as much as possible, and a large dose concentrates on the affected part. However, actually, a position of an affected part from a body surface is periodically changed due to respiration of a patient. Radiation respiration synchronized irradiation is performed in which a change of a position of an affected part due to respiration is detected, a cycle of the position change is measured, and the affected part is irradiated with radiation in synchronized with the cycle of the position change of the affected part. An example of the respiration synchronized irradiation is disclosed in JP-A-7-303710. In JP-A-7-303710, an ultrasonic tomographic apparatus generates a tomographic image of an affected part vicinity on the basis of an ultrasonic signal received by a probe provided on a body surface of a patient, and an image processing apparatus creates information indicating a cyclic position change of the affected part. The affected part is irradiated with a particle beam at a timing at which a position of the affected part is not changed in this cycle. JP-A-7-303710 also discloses that information indicating a cyclic position change of an affected part is created on the basis of an output signal from a respiration monitor instead of the ultrasonic tomographic apparatus.
JP-A-2015-157003 discloses a charged particle beam irradiation method in which an affected part of cancer is divided into a plurality of layers from a body surface in an irradiation direction of an ion beam, scanning with a thin ion beam is performed, and thus the ion beams are applied to a plurality of irradiation spots which are irradiation positions in each layer. Movement of an ion beam to a neighboring irradiation spot in each layer is performed by a scanning control device controlling a scanning electromagnet which changes a position of the ion beam.
In a depth direction of a human body, a dose distribution as illustrated in FIG. 6 of JP-A-2015-157003 is shown, a dose becomes the maximum at a Bragg peak, and the dose distribution is rapidly reduced at a depth exceeding the Bragg peak. Cancer therapy using an ion beam uses the property that a dose becomes the maximum at a Bragg peak, and the dose is rapidly reduced at a depth exceeding the Bragg peak.